1. Field of the Invention
The present invention relates to a zoom lens adapted for use in a television camera.
2. Description of Related Art
FIG. 3 is a block diagram showing the arrangement of a zoom driving part of a conventional zoom lens. A signal output of a zoom demand (operation) part 1 is connected through a limit circuit 2 to the positive terminal of a subtracter 3 and an amplifier 4 one after the other. The output of the amplifier 4 is connected to a motor 6 which is arranged to drive a zooming lens 5. A speed detecting means 7 for detecting the angular velocity of the motor 6, such as a tachometer generator, is connected to the motor 6. A signal outputted from the speed detecting means 7 is fed back to the negative terminal of the subtracter 3. A so-called speed feedback control system is thus formed by the subtracter 3, the amplifier 4, the motor 6 and the speed detecting means 7. The zooming lens 5 is controlled and driven by this speed feedback control system.
An absolute position detecting means 8, such as a potentiometer, arranged to detect the absolute position of the zooming lens 5 is also connected to the motor 6. The output of the absolute position detecting means 8 is connected to the limit circuit 2.
In the above-stated arrangement, the relation of a signal output of the absolute position detecting means 8 to the position of the zooming lens 5 is uniquely determined. Therefore, it is possible to construct the limit circuit 2 in such a way that the value of the input to the positive terminal of the subtracter 3 becomes "0" at a mechanical end point of a zoom lens driving mechanism according to a predetermined speed reduction curve.
Therefore, the zooming lens 5 can be driven and moved to a desired position by arbitrarily operating the zoom demand part 1 the instant electric power is supplied to the zoom lens. When the zooming lens 5 comes near the mechanical end point of the zoom lens driving mechanism, the limit circuit 2 acts to make a speed command signal "0" irrespective of the intention of the operator. The limit circuit 2 thus enables the operator to operate the zoom lens without taking heed to the possibility of having the zooming lens 5 bumping against the mechanical end point.
Meanwhile, during recent years, a higher degree of control has come to be required with respect to lens systems, because there have arisen various demands. The demands, for example, include a demand for a function of eliminating changes in angle of view due to focusing by correcting such changes by zooming. Another demand is for a severe control over the focal length of a lens in use, for example, at a virtual studio or the like where actual video images are broadcasted in combination with computer graphics. These demands require a higher degree of position control than the conventional control level. Therefore, to meet such requirements, lens systems have been developed to include therein built-in arithmetic units such as microcomputers and to control lens positions by means of the built-in arithmetic units.
For carrying out position control with a higher degree of precision, these lens systems are arranged, in most cases, to use encoders as position detectors in place of potentiometers which have conventionally been used as main position detectors, because use of the encoder makes processes to be carried out with the microcomputer easier.
However, in a case where the encoder in use is arranged to detect an absolute position, i.e., in the event of a so-called absolute-type encoder, the encoder is larger in size, more expensive and has lower resolution than a so-called incremental-type encoder which detects a relative position. The incremental-type encoder is small in size and has a high resolution. However, with the incremental-type encoder used, it is impossible to decide the absolute position of the lens at the time when a power supply is turned on. Hence, the use of the incremental-type encoder necessitates, before the lens is operated, a process of deciding the absolute position of the lens by driving the lens to a datum position at the time when a power supply is turned on. Since the lens must be driven to a datum position, as its absolute position is unknown at the time of turning-on of a power supply, the method of using an incremental-type encoder takes time before the lens reaches the datum position. Besides, the datum position tends to vary due to some factor such as backlash.